This application claims priority from Belgium application No. 09800526, filed Jul. 10, 1998.
The present invention relates to hollow bodies based on random propylene terpolymers. It relates more particularly to biaxially oriented hollow bodies possessing good optical and mechanical properties obtained from depolymerized random propylene terpolymers.
The present invention also relates to a process for the manufacture of these hollow bodies and more particularly the manufacture of biaxially oriented hollow bodies by injection stretch blow moulding of preforms, also known as parisons.
The ceaselessly growing demand for plastic hollow bodies of high quality possessing the transparency of glass has resulted in intensive research, especially in the polypropylene (PP) sector. However, the use of such a material leads to problems of control of manufacture. Manufacturers of equipment, such as Bekum and Nissei ASB, have developed devices which overcome the difficulties encountered with the machines constructed previously (problems of control of the manufacturing cycle) and which allow the production of biaxially oriented PP (BOPP) hollow bodies of high quality by an injection stretch blow moulding cycle. The PP resins used for this application are, on the one hand, homopolymers which are rigid but translucent and, on the other hand, random copolymers with ethylene which exhibit a wide range of melting temperatures and good optical properties but an inferior rigidity. There therefore still exists a market demand for hollow bodies based on polypropylene resins exhibiting an optimum compromise between the mechanical and optical properties and the processability.
Application DE-A-2,827,764 discloses a process for the preparation of a random propylene terpolymer, intended in particular for the manufacture of heat-sealable sheets, by copolymerization of propylene with ethylene and a higher 1-alkene and then degradation (depolymerization) of the polymer obtained by extrusion in the molten state in the absence of oxygen and of free radical generators.
U.S. Pat. No. 5,436,041 claims a printable transparent biaxially oriented multilayer sheet, both faces of which are sealable; the central layer is composed of a PP depolymerized by peroxides (preferred degradation factor: from 4 to 8) and one of the faces can be composed of a random C3-C2-C4 terpolymer which can be depolymerized by peroxides (preferred degradation factor: from 6 to 10).
A random propylene terpolymer degraded (depolymerized) by free radical generators which is very particularly suitable for the production of transparent biaxially oriented hollow bodies has now been found.
To this end, the present invention relates to biaxially oriented hollow bodies obtained by employing a random propylene terpolymer which comprises from 83 to 97.5% by weight of monomer units derived from propylene, from 0.5 to 5% by weight of monomer units derived from ethylene and from 2 to 12% by weight of monomer units derived from 1-butene, which has a melting temperature of 120 to 150xc2x0 C. (measured according to ASTM Standard D 3418) and which is degraded by a free radical generator, the degradation factor A being from 1.5 to 10 and the said factor A being defined as being the ratio MFIg/MFi, in which MFIg is the melt flow index of the degraded terpolymer and is between 3 and 30 g/10 min and MFIi is the melt flow index of the initial terpolymer and is between 2 and 15 g/10 min.
The melt flow index (MFI) is measured at 230xc2x0 C. under a load of 2.16 kg and is expressed in g/10 min (ASTM Standard D 1238).